The present invention relates to an image formation apparatus such as a copying machine based on electrophotography.
Conventionally, an image formation apparatus based on electrophotography is equipped with a developing unit in the vicinity of the photoreceptor drum surface. A developing agent is contained in the developing unit. The developing unit supplies the developing agent to the photoreceptor drum surface for forming an electrostatic latent image on the surface. When the developing agent is of two-component type including components of thermoplastic resin powder toner and magnetic fine particle carrier, the toner and the carrier are mixed together in the developing unit so that they induce frictional electric charge in each other, which causes the toner to adhere electrostatically to the carrier surface. The developing unit has an opening at a position facing the photoreceptor drum surface. A magnet roller is positioned in the developing unit so that the magnet roller faces to the photoreceptor drum surface through the opening. The carrier has toner adhering thereto, which sticks magnetically to the magnet roller surface. Thus, the developing agent made up of toner and carrier forms a magnetic brush between the photoreceptor drum and the magnet roller, so that the toner alone adheres to the electrostatic latent image formed on the photoreceptor drum. The potential for forming an electrostatic latent image is realized by charging the photoreceptor drum surface with an electric charger. Therefore, a corona discharge by the electric charger has an opposite polarity to the polarity of the potential of the friction-charged toner.
However, in the above conventional image formation apparatus using a two-component developing agent, the toner constituting the developing agent will become unevenly charged as the toner deteriorates with age. The toner may be charged to a potential of the reverse polarity. In the developing process, such defective toner sticks to an area other than the area having an electrostatic latent image on the photoreceptor drum. Consequently, the toner is transferred to the blank area or the background of an image on copy paper, and an image of poor quality results.